Flotation plants are used to an ever-increasing extent in the purification of water, both drinking water and sewage water, with the intention of increasing the rate at which the water flows through the purification plant and therewith also enable the size of the necessary tanks and thus the purification plant as a whole to be reduced. With regard to drinking water, the requirements of the end product are essentially unitary and the construction of a purification plant is essentially determined by the nature of the water source or procurement. When the water source is a lake or river, coarse particles are normally separated from the water in a screening chamber, whereafter the water is purified chemically and biologically by adding thereto precipitation and flocculating agents in a flocculating chamber. The precipitated particles are then separated from the water, by allowing the particles to settle to the bottom of a sedimentation tank gravitationally. Different types of particles will have different densities and therewith different settling rates. In order for particles of low densities, "light particles", to have time to settle as the water passes through a sedimentation tank, the tank must be given a large surface area so to provide the long period of time taken for such light particles to settle.
Particle separation times can be greatly reduced by flotation processes, in which there are added to the water microscopic air bubbles to which particles or particle flocs in the water adhere. These bubbles also contribute in forming particle flocs and in maintaining floc coherency. Air bubbles rise quickly to the surface of the water, carrying with them the adhered particles and particle flocs, these flocs forming a stable coating of slime on the surface of the water, which can be removed with the aid of scrapers or by intermittently raising the water level in the flotation tank so that the slime coating will flow into a slime chute provided at one edge of the tank.
The microscopic air bubbles used in flotation processes have a diameter of 30-80 .mu.m and cannot be produced by simply injecting air directly into water, for instance. Microscopic air bubbles intended for drinking water flotation processes are normally produced by releasing pressurized air into clean water from a pressure vessel. The highest possible degree of saturation is sought for in this regard. This water, normally referred to as dispersion water, is conducted to an inlet for water that has undergone a flocculation process, so-called flocculated water, and which is situated at the bottom of the flotation tank, while maintaining the high pressure of said water, and is there delivered to the flocculated water through separate nozzles or jets which are constructed to generate an instantaneous decrease in pressure to a level at which the air dissolved in the water is released in the form of microscopic bubbles and forms a dispersion of air in the water.
Those pressure vessels used to produce pressurized dispersion water that has been saturated with air to the greatest possible extent are normally kept half full with water with the aid of control equipment and associated water level sensor means, and the incoming and outgoing flows of water are located beneath the surface of the water in the pressure vessel. Compressed air is delivered to the air-filled space above the water surface and the vessel has a relatively large size so as to obtain a large surface contact between air and water. In another known design, the diameter of the pressure vessel has been reduced, said pressure vessel often being man high, and the thus reduced air/water contact surface is compensated for by delivering the water through a nozzle provided in the vessel side wall in the air space, through which nozzles the water is injected onto the opposing vessel wall, therewith disintegrating the water jets to some extent.
With regard to water purification, the flotation method takes a much shorter time to effect and this shorter throughflow time enables purification plants to be made smaller with retained capacity.
The reduction in the space requirements of such purification plants opens new avenues of use. Such development requires all components to be given smaller dimensions and preferably also improved properties. This also applies to the dispersion water preparing unit, in which water that has already been cleaned or purified is used and recycled. An increase in the degree of air saturation in the dispersion water reduces the need of recycled water and therewith improves the efficiency of the purification plant as a whole.